Clock generation for timing communications with ranks of memory devices

ABSTRACT

A memory controller includes a clock generator to generate a first clock signal and a timing circuit to generate a second clock signal from the first clock signal. The second clock signal times communications with any of a plurality of memory devices in respective ranks, including a first memory device in a first rank and a second memory device in a second rank. The timing circuit is configured to adjust a phase of the first clock signal, when the memory controller is communicating with the second memory device, based on calibration data associated with the second memory device and timing adjustment data associated with feedback from at least the first memory device.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/990,370, filed May 29, 2013, which was a U.S. National StageApplication filed under 35 U.S.C. §371 of PCT Patent Application No.PCT/US2011/059851, filed Nov. 9, 2011, which claims priority to U.S.Provisional Patent Application No. 61/417,845, filed Nov. 29, 2010, eachof which are hereby incorporated by reference in their entireties.

TECHNICAL FIELD

The disclosed embodiments relate generally to data communicationsbetween memory controllers and memory devices, and more particularly, togenerating clock signals for timing data communications between a memorycontroller and memory devices.

BACKGROUND

The timing of data communications between a memory controller and memorydevices presents significant engineering challenges. For example,voltage and temperature variation during operating can cause a previouscalibration to become obsolete. Frequent calibrations, however, causesystem delays. Also, inclusion of a PLL or DLL on the memory devices isnot desirable, due to power, die area, and system delay issues.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A-1C are block diagrams illustrating memory systems that include amemory controller and memory devices arranged in ranks, in accordancewith some embodiments.

FIGS. 2A-2C are block diagrams illustrating circuitry in a memorycontroller for timing communications with memory devices in accordancewith some embodiments.

FIG. 3A is a circuit diagram illustrating circuitry in a memorycontroller for timing data transmission to memory devices in respectiveranks, in accordance with some embodiments.

FIG. 3B is a circuit diagram illustrating circuitry in a memorycontroller for timing the receipt of data from memory devices inrespective ranks, in accordance with some embodiments.

FIGS. 4A and 4B are block diagrams illustrating a plurality of storageelements for storing calibration data in accordance with someembodiments.

FIG. 5 is a circuit diagram of edge tracking circuitry implemented usingbang-bang CDR logic in accordance with some embodiments.

FIG. 6A is a circuit diagram of data communication circuitry in a memorycontroller in accordance with some embodiments.

FIG. 6B is a circuit diagram of data communication circuitry in a memorydevice in accordance with some embodiments.

FIGS. 7A-7D are flow diagrams illustrating methods of controlling memorydevices in accordance with some embodiments.

Like reference numerals refer to corresponding parts throughout thedrawings.

DESCRIPTION OF EMBODIMENTS

In some embodiments, a memory controller includes a clock generator togenerate a first clock signal and a timing circuit to generate a secondclock signal from the first clock signal. The second clock signal timescommunications with any of a plurality of memory devices in respectiveranks, including a first memory device in a first rank and a secondmemory device in a second rank. The timing circuit is configured toadjust a phase of the first clock signal, when the memory controller iscommunicating with the second memory device, based on calibration dataassociated with the second memory device and timing adjustment dataassociated with feedback from at least the first memory device.

In some embodiments, a memory controller includes a clock generator togenerate a first clock signal and a timing circuit to generate a secondclock signal from the first clock signal. The second clock signal timescommunications with any of a plurality of memory devices in respectiveranks, including a first memory device in a first rank and a secondmemory device in a second rank. The timing circuit includes a firstphase adjuster to generate an intermediate clock signal by adjusting aphase of the first clock signal; a plurality of respective storageelements, each to store calibration data associated with a respectivememory device in a respective rank and configured to be selectivelycoupled to the first phase adjuster when the memory controller is tocommunicate with the respective memory device in the respective rank;and a second phase adjuster to generate the second clock signal byadjusting a phase of the intermediate clock signal based on timingadjustment data associated with feedback from one or more of theplurality of memory devices in the respective ranks. The first phaseadjuster is configured to adjust the phase of the first clock signalbased on the calibration data stored in the selectively coupled storageelement.

In some embodiments, a memory controller includes a clock generator togenerate a first clock signal and a first phase adjuster to generate asecond clock signal by adjusting a phase of the first clock signal. Thesecond clock signal times communications with any of a plurality ofmemory devices in respective ranks. The memory controller also includesa plurality of respective storage elements, each to store calibrationdata associated with a respective memory device in a respective rank andconfigured to be selectively coupled to the first phase adjuster whenthe memory controller is to communicate with the respective memorydevice in the respective rank. The first phase adjuster is configured toadjust the phase of the first clock signal based on the calibration datastored in the selectively coupled storage element. The memory controlleralso includes calibration circuitry to perform calibration for a firstmemory device in a first rank and, in response, to determine an amountby which to adjust calibration data associated with the first memorydevice in a first storage element of the plurality of storage elements.The memory controller further includes adjustment circuitry to adjustcalibration data associated with the second memory device in a secondstorage element of the plurality of respective storage elements by thedetermined amount.

In some embodiments, a method of controlling memory devices is performedat a memory controller coupled to memory devices in a plurality ofranks, including a first memory device in a first rank and a secondmemory device in a second rank. The method includes generating a firstclock signal and adjusting a phase of the first clock signal to generatea second clock signal. The phase of the first clock signal is adjustedbased on calibration data associated with the second memory device andtiming adjustment data associated with feedback from at least the firstmemory device. The method also includes timing communications with thesecond memory device, in accordance with the second clock signal.

In some embodiments, a method of controlling memory devices is performedat a memory controller coupled to memory devices in a plurality ofranks, including a first memory device in a first rank and a secondmemory device in a second rank. The method includes performing acalibration of first timing for communication between the memorycontroller and the first memory device and performing a calibration ofsecond timing for communication between the memory controller and thesecond memory device. The method also includes communicating with thefirst memory device in accordance with the first timing and, whilecommunicating with the first memory device, determining timingadjustments based on feedback from the first memory device and modifyingthe first timing based on the timing adjustments. The method furtherincludes communicating with the second memory device in accordance withthe second timing as modified by the timing adjustments and, whilecommunicating with the second memory device, determining additionaltiming adjustments based on feedback from the second memory device andmodifying the second timing based on the additional timing adjustments.

In some embodiments, a memory system includes a plurality of memorydevices in respective ranks, including a first memory device in a firstrank and a second memory device in a second rank. The memory system alsoincludes a memory controller that includes a clock generator to generatea first clock signal and a timing circuit to generate a second clocksignal from the first clock signal. The second clock signal timescommunications with any of the plurality of memory devices. The timingcircuit is configured to adjust a phase of the first clock signal, whenthe memory controller is communicating with the second memory device,based on calibration data associated with the second memory device andtiming adjustment data associated with feedback from at least the firstmemory device.

Reference will now be made in detail to various embodiments, examples ofwhich are illustrated in the accompanying drawings. In the followingdetailed description, numerous specific details are set forth in orderto provide a thorough understanding of the present inventions. However,the present inventions may be practiced without these specific details.In other instances, well-known methods, procedures, components, andcircuits have not been described in detail so as not to unnecessarilyobscure aspects of the embodiments.

FIG. 1A is a block diagram illustrating a memory system 100 inaccordance with some embodiments. The memory system 100 includes amemory controller 102 coupled to a plurality of memory devices 104,which are arranged in ranks 106 a through 106 n as shown. In someembodiments, the memory system 100 includes only two ranks 106 (e.g.,ranks 106 a and 106 b). In some embodiments, the memory system 100includes three ranks 106, or four ranks 106 or more.

A first collection of signal lines 108-1, referred to as a lane 108-1,couples the memory controller 102 to a first memory device 104 in eachof the ranks 106 a through 106 n. The lane 108-1 includes signal linesfor transferring data, referred to as data lines, between the memorycontroller 102 and the memory devices 104 that are coupled to the lane108-1. The memory devices 104 coupled to the lane 108-1 are referred toas a slice 110-1. The number of data lines in the lane 108-1 equals thewidth of each memory device 104, where the term width refers to thenumber of bits in data words written to and read from the memory device104. For example, if each memory device 104 is ×8 (“by 8”), meaning thatits data words have eight bits, then the lane 108-1 includes eightsignal lines for transferring data between the memory controller 102 andmemory devices 104; such a lane 108-1 is referred to as a byte lane. Inanother example, each memory device 104 is ×4 (“by 4”), meaning that itsdata words have four bits, and the lane 108-1 includes four signal linesfor transferring data between the memory controller 102 and memorydevices 104; such a lane 108-1 is referred to as a nibble lane. Otherwidths and corresponding numbers of data lines in the lane 108-1 arepossible.

In some embodiments, the lane 108-1 includes other signal lines inaddition to the data lines. For example, the lane 108-1 may include oneor more additional signal lines for transmitting data strobes associatedwith the data. The lane 108-1 also includes one or more signal lines forproviding feedback from the memory devices 104 to the memory controller102. For example, the lane 108-1 includes a signal line for transmittingan error detection code (EDC) signal from the memory devices 104 to thememory controller 102. Optionally, feedback is provided by strobesignals (e.g., read strobes) from the memory devices 104.

Additional lanes 108 couple the memory controller 102 to additionalrespective memory devices 104 in each of the ranks 106 a through 106 n,as described for the lane 108-1. For example, the lane 108-2 couples thememory controller 102 to a slice 110-2 that includes a second respectivememory device 104 in each of the ranks 106 a through 106 n, and the lane108-3 couples the memory controller 102 to a slice 110-3 that includes athird respective memory device 104 in each of the ranks 106 a through106 n. Furthermore, the memory system 100 includes additional signallines (not shown) between the memory controller 102 and memory devices104 including, for example, clock lines to transmit clock signals fromthe memory controller 102 to the memory devices 104 and signal lines(e.g., C/A busses) to transmit command and address information from thememory controller 102 to the memory devices 104.

At any given time during operation of the memory system 100, one of theranks 106 a through 106 n may be selected and the other ranksdeselected. (Alternatively, all ranks 106 a through 106 n aredeselected, and the system 100 is in a standby or power-down mode inwhich no communication of data occurs between the memory controller 102and memory devices 104.) The memory controller 102 writes data to andreads data from the memory devices 104 in the selected rank 106 and notthe memory devices 104 in the deselected ranks 106. For example, formemory devices 104 in deselected ranks 106, the input/output pinscoupled to the lanes 108 are tristated. (The term pin as used hereinincludes pins as well as solder balls, lands, and other contacts on anappropriate semiconductor package.). The memory controller 102 switchesbetween ranks 106 by deselecting the selected rank 106 (e.g., rank 106a) and selecting a new rank (e.g., rank 106 b). After performing thisswitch, the memory controller 102 writes data to and reads data from thememory devices 104 in the newly selected rank 106 (e.g., rank 106 b) andnot the memory devices 104 in the deselected ranks (e.g., rank 106 a andany other ranks).

Switching between ranks presents challenges for calibration and timing.While inclusion of a delay-locked loop (DLL) or phase-locked loop (PLL)on each of the memory devices 104 can at least partially resolve timingissues, inclusion of a DLL or PLL on the devices 104 is not desirable.DLLs and PLLs consume more power than is desirable for low-powerapplications. While DLLs or PLLs of memory devices 104 in deselectedranks 106 can be powered down, they require time to power up and lockwhen their rank 106 is selected, resulting in system delays. Thus, insome embodiments, the memory devices 104 include neither DLLs nor PLLs.

To account for timing variation resulting from process variation (i.e.,variation in the manufacturing process) between memory devices 104 inrespective ranks 106 in a slice 110, the memory system 100 performs arank-specific calibration for each rank 106 and stores calibration datafor each memory device 104 that is subsequently used to timecommunications (e.g., reads or writes) with respective memory devices104. This calibration data may not be sufficient to provide adequatetiming margins, however, because of voltage and temperature changes overtime. For example, the voltage(s) supplied to memory devices 104 in arank 106 and the temperatures of the memory devices 104 in the rank 106may drift in the time between calibration of the rank 106 andcommunication between the memory controller 102 and the rank 106,resulting in a loss of timing margin and thus failed communication.

To account for voltage and temperature variation, timing ofcommunications between the memory controller 102 and devices 104 in aselected rank 106 is adjusted based on feedback received from thedevices 104 as well as on calibration data for the devices 104.Furthermore, in some embodiments, the devices 104 in respective ranks106 in a given slice 110 are situated in physical proximity, such thatthey experience substantially the same voltage and temperaturevariation. In these embodiments, timing adjustments based on feedbackfrom a first device 104 in a first rank 106 may be applied tocommunications with a second device 104 situated in a second rank 106 inthe same slice 110 as the first device 104: the feedback from the firstdevice 104 provides information about voltage and temperature variationexperienced by both the first and second devices 104, because of thephysical proximity of the first and second devices 104. Thus, in someembodiments, when the controller 102 deselects a first rank 106 andselects a second rank 106, communications with the second rank 106 aretimed using both calibration data for devices 104 in the second rank 106and feedback previously received from devices 104 in the first rank 106,as well as feedback newly received from devices 104 in the second rank106.

FIG. 1B is a cross-sectional view of a memory system 120 in which memorydevices 104 in respective ranks 106 in a slice 110 are located inphysical proximity such that they experience substantially the samevoltage and temperature variation. The memory system 120, which is anexample of the memory system 100 (FIG. 1A), includes a memory module 122coupled to a memory controller 102. The memory module 122 includes afirst memory device 104 a connected to a first side of a card 124 and asecond memory device 104 b connected to a second side of the card 124.The first memory device 104 a is part of a first rank 106 (e.g., 106 a,FIG. 1A) of memory devices 104 on the first side of the card 124 and thesecond memory device 104 b is part of a second rank 106 (e.g., 106 b,FIG. 1A) of memory devices 104 on the second side of the card 124. Thesystem 120 thus includes two ranks 106 of memory devices 104. The firstand second memory devices 104 a and 104 b are situated back-to-back onthe module 122 and thus are in close physical proximity.

A socket 132 connects the module 122 to a circuit board 138, to whichthe memory controller 102 also is connected. A signal line 134 in thecircuit board 138 and a signal line 126 in the module 122 togetherconstitute a data line that connects data pins (not shown) on the memorydevices 104 a and 104 b to a data pin 136 (shown as a solder ball) onthe controller 102. A contact 130 in the socket 132 and a contact 128 onthe card 124 connect the signal line 134 to the signal line 126, asshown. The data line formed by the signal lines 134 and 126 is part of alane 108 (e.g., lane 108-1, FIG. 1); the memory devices 104 a and 104 bare thus in the same slice 110 (e.g., slice 110-1, FIG. 1).

FIG. 1C illustrates a cross-sectional view of another memory system 150in which memory devices 104 in respective ranks 106 in a slice 110 arelocated in physical proximity such that they experience substantiallythe same voltage and temperature variation. The memory system 150 isanother example of the memory system 100 (FIG. 1A). In the system 150,semiconductor package 152 contains two memory devices 104 c and 104 d ina stacked configuration. The package 152, also referred to as amulti-chip module 152, is coupled to a memory controller 102 via acircuit board 160, to which the package 152 and controller 102 areconnected. The first memory device 104 c is part of a first rank 106(e.g., 106 a, FIG. 1A) of memory devices 104 and the second memorydevice 104 d is part of a second rank 106 (e.g., 106 b, FIG. 1A) ofmemory devices 104. For example, the system 150 includes multiplepackages 152, each of which includes a memory device 104 in each of therespective ranks. The package 152 is shown with two stacked memorydevices 104 c and 104 d, and the system 150 is correspondingly shown ashaving two ranks 106 of memory devices 104. In some embodiments,however, the package 152 includes more than two stacked memory devices104 (e.g., includes three or four stacked memory devices 104) and thesystem 150 correspondingly includes more than two ranks 106 (e.g.,includes three or four ranks 106).

A signal line 158 in the circuit board 160 and a signal line 154 in thepackage 152, connected via a pin 156 on the package 152, togetherconstitute a data line that connects data pads (not shown) on the memorydevices 104 c and 104 d (which in this example are dice housed in thepackage 152, as opposed to the packaged devices 104 a and 104 b of FIG.1B) to the data pin 136 on the controller 102. The data line formed bythe signal lines 158 and 154 is part of a lane 108 (e.g., lane 108-1,FIG. 1); the memory devices 104 c and 104 d are thus in the same slice110. In some embodiments, the signal line 154 in the package 152 isimplemented, at least in part, using through-silicon via technology.

Attention is now directed to circuitry in the memory controller 102 fortiming communications (e.g., reads and writes) with memory devices 104.In FIG. 2A, a clock generator 202 generates a first clock signal 204that includes a plurality of phase vectors (e.g., evenly spaced phasevectors). A timing circuit 206 receives the first clock signal 204 andgenerates a second clock signal (“Ck”) 214 from the first clock signal204 by adjusting the phase of the first clock signal 204. The memorycontroller 102 uses the second clock signal 214 to time communicationswith memory devices 104, as explained below, for example, with respectto FIGS. 3A-3B.

Specifically, the clock generator 202 provides the first clock signal204 to a phase mixer 208 in the timing circuit 206. The phase mixer 208is coupled to a plurality of storage elements 218, each of whichincludes a respective one of a plurality of registers 220 a through 220n, where n is an integer corresponding to the number of ranks 106 in thesystem 100 (FIG. 1A). Each register 220 stores calibration data 216 fora memory device 104 in a corresponding rank 106; the memory devices forwhich the registers 220 store calibration data 216 are all in the sameslice 110. For example, the register 220 a stores calibration data for amemory device 104 (e.g., 104 a, FIG. 1B, or 104 c, FIG. 1C) in the rank106 a and the register 220 b stores calibration data for a memory device104 (e.g., 104 b, FIG. 1B, or 104 d, FIG. 1C) in the rank 106 b, withboth memory devices being situated in the same slice 110 (e.g., slice110-1, on the lane 108-1). When the memory controller 102 selects one ofthe ranks 106 and communicates with memory devices 104 in the selectedrank, it provides a signal specifying the selected rank 222 to theplurality of storage elements 218. In response, the register 220corresponding to the selected rank 222 is coupled to the phase mixer 208and the calibration data 216 stored in the register 220 corresponding tothe selected rank 222 is provided to the phase mixer 208, which adjuststhe phase of the first clock signal 204 in accordance with thecalibration data 216 for the selected rank 222 and thereby generates anintermediate clock signal 210. Each of the registers 220 a through 220 nthus can be selectively coupled to the phase mixer 208. The calibrationdata 216 serves as an input to the phase mixer 208 to specify the amountby which to adjust the phase of the first clock signal 204 (e.g., byspecifying how to mix phase vectors of the first clock signal 204).

The phase mixer 208 provides the intermediate clock signal 210 to adigitally controlled delay line (DCDL) 212, which adjusts the phase ofthe intermediate clock signal 210 by delaying the intermediate clocksignal 210 by a programmable amount. The output of the DCDL 212 is thesecond clock signal 214, which the memory controller 102 uses to timecommunications with a memory device 104 in the selected rank 222 (e.g.,as described below with respect to FIGS. 3A and 3B). The amount by whichthe DCDL 212 delays the intermediate clock signal 210 is determined bytiming adjustment data 224 received from edge tracking circuitry 226,which generates the timing adjustment data 224 based on feedbackreceived from memory devices 104 in a slice 110. The timing adjustmentdata 224 is thus derived from the feedback. In some embodiments, thefeedback is provided by an EDC signal 228 transmitted over a dedicatedsignal line of the lane 108. At any given time, the feedback is receivedfrom the memory device 104, situated in the slice 110, which is in theselected rank 106. In some embodiments, when the memory controllerswitches the selected rank 106, the edge tracking circuitry 226continues to receive feedback (e.g., the EDC signal 228) with no delaysbeyond those associated with switching ranks 106. The edge trackingcircuitry 226 is implemented using clock-and-data-recovery (CDR) logic,an example of which is described below with respect to FIG. 5.

In some embodiments, the edge tracking circuitry 226 is coupled to theplurality of storage elements 218 and provides timing adjustment data tothe storage elements 218 to update the calibration data, as describedbelow with respect to FIGS. 4A and 4B. For example, the edge trackingcircuitry 226 accumulates the timing adjustment data 224 andperiodically provides the accumulated timing adjustment data 224 to thestorage elements 218 as periodic updates to the calibration data.

In some embodiments, the DCDL 212 is replaced with a second phase mixer246, as illustrated for the timing circuit 240 in FIG. 2B. Also, thephase mixer 208 is replaced with a phase mixer 242 that provides aplurality of phase vectors 244 (e.g., evenly spaced phase vectors) asthe intermediate clock signal. The second phase mixer 246 receives thephase vectors 244 and mixes them, in accordance with the timingadjustment data 224, to produce the second clock signal 214.

The timing circuit 206 (FIG. 2A) or 240 (FIG. 2B) includes calibrationcircuitry 230 that generates the calibration data 216 stored in theregisters 220. For example, when the memory controller 102 enters acalibration mode, the calibration circuitry 230 transmits a calibrationdata pattern 232 to a memory device 104 in a selected rank 106,determines the calibration data 216 for the memory device 104 based onfeedback from the memory device 104 (e.g., based on the EDC signal 228),stores the calibration data 216 in the corresponding register 220, andrepeats the process for each rank 106 and corresponding register 220.

In some embodiments, the memory controller 102 includes an instance(i.e., a separate instance) of the timing circuit 206 (FIG. 2A) or 240(FIG. 2B) for each lane 108 in the system 100. Alternatively, the memorycontroller 102 includes an instance of the timing circuit 206 (FIG. 2A)or 240 (FIG. 2B) for each data line in each lane 108, or for a subset ofthe data lines in each lane 108. In other alternatives, the memorycontroller 102 includes respective instances of the phase mixer 208 or242, the DCDL 212 or phase mixer 246, and the registers 220 a through220 n for each data line in each lane 108, and includes a singleinstance of the edge tracking circuitry 226 and calibration circuitry230 for each lane 108.

The timing circuits 206 (FIG. 2A) and 240 (FIG. 2B) thus allow thememory controller 102 to time communications with a respective memorydevice 104 based on calibration data 216 for the respective memorydevice 104 as well as feedback received from the respective memorydevice 104 and potentially from one or more other memory devices 104.For example, when the memory controller 102 switches from communicatingwith a first memory device 104 in a first rank 106 to communicating witha second memory device 104 in a second rank 106 (e.g., by deselectingthe first rank 106 and selecting the second rank 106), the memorycontroller 102 times initial communications with the second memorydevice 104 based on the calibration data 216 from the register 220corresponding to the second memory device 104 and also on the timingadjustment data 224 that the edge tracking circuitry 226 generated basedon the feedback received from the first memory device 104. Ascommunications with the second memory device 104 continue, the edgetracking circuitry 226 will update the timing adjustment data 224 basedon newly-received feedback from the second memory device 104, such thatthe timing adjustment data 224 is now based on feedback from both thefirst and second memory devices 104. In some embodiments in which thesystem 100 includes three or more ranks 106, sufficiently rapidswitching between ranks 106 results in the timing adjustment data 224being based on feedback from at least three of the ranks 106, or fromall of the ranks 106.

The timing circuits 206 (FIG. 2A) and 240 (FIG. 2B) perform two-stagephase adjustment of the first clock signal 204, with the phase mixers208 or 242 being a first stage and the DCDL 212 or phase mixer 246 beinga second stage. In some embodiments, however, timing circuitry 260performs single-stage phase adjustment of the first clock signal 204, asillustrated in FIG. 2C. The timing circuitry 260 omits the DCDL 212(FIG. 2A) and phase mixer 246 (FIG. 2B). Instead, the clock signal 210provided by the phase mixer 208 is used to time communications withmemory devices 104. In some embodiments, in addition to generatingcalibration data 216, the calibration circuitry 230 generates timingadjustment data, analogous to the timing adjustment data 224 generatedby the edge tracking circuitry 226 (FIGS. 2A-2B), and periodicallyadjusts the calibration data stored in the registers 220 based on thetiming adjustment data.

FIG. 3A is a circuit diagram illustrating circuitry 300 in a memorycontroller 102 for timing data transmission to memory devices 104 inrespective ranks 106 in a slice 110, in accordance with someembodiments. In the circuitry 300, a transmitter 306 receives data 308and transmits it as output data (“Data Out”) 310. The transmitter 306 isconnected to a data line of a lane 108 and transmits the output data310, via the data line, to any of the memory devices 104 in respectiveranks 106 in the slice 110 corresponding to the lane 108. Thetransmitter 306 is synchronized to a clock signal 304 (e.g., Ck 214,FIGS. 2A-2B, or clock signal 210, FIG. 2C) provided by a timing circuit302, examples of which include the timing circuits 206 (FIG. 2A), 240(FIG. 2B), and 260 (FIG. 2C). The timing circuit 302 receives a firstclock signal 204 from the clock generator 202, as described above withrespect to FIGS. 2A-2C.

FIG. 3B is a circuit diagram illustrating circuitry 320 in a memorycontroller 102 for timing the receipt of data from memory devices 104 inrespective ranks 106, in accordance with some embodiments. In thecircuitry 320, a sampling circuit 312 samples input data (“Data In”) 314and provides the sampled data 316 to other circuitry in the memorycontroller 102 for processing. The sampling circuit 312 is connected toa data line of a lane 108 and receives the input data 314, via the dataline, from any of the memory devices 104 in respective ranks 106 in theslice 110 corresponding to the lane 108. The sampling circuit 312 issynchronized to a clock signal 304 (e.g., Ck 214, FIGS. 2A-2B, or clocksignal 210, FIG. 2C) provided by a timing circuit 302, examples of whichinclude the timing circuits 206 (FIG. 2A), 240 (FIG. 2B), and 260 (FIG.2C). The timing circuit 302 receives a first clock signal 204 from theclock generator 202, as described above with respect to FIGS. 2A-2C.

FIG. 4A is a block diagram illustrating a plurality of storage elements400 for storing calibration data in accordance with some embodiments.The plurality of storage elements 400 is an example of the plurality ofstorage elements 218 (FIGS. 2A-2C). As described with respect to FIGS.2A-2C, the plurality of storage elements 400 includes registers 220 athrough 220 n, each corresponding to a respective rank 106. Calibrationdata 216 (e.g., as received from the calibration circuitry 230, FIGS.2A-2C) for memory devices 104 in the respective ranks 106 a through 106n are stored in the corresponding registers 220 a through 220 n. Thespecified rank 222 (e.g., the rank that is currently selected by thememory controller 102) is provided to an address decoder 402, whichselects the corresponding register 220. The corresponding register 220is thereby selectively coupled to the phase mixer 208 (FIGS. 2A and 2C)or 242 (FIG. 2B) and provides its calibration data 216 to the phasemixer 208 or 242. Coupled to each register 220 a through 220 n is anadder 404 a through 404 n, which together constitute adjustmentcircuitry 406 used to update the calibration data 216 stored in theregisters 220 a through 220 n. The calibration data 216 is updated, forexample, to account for voltage and temperature drift since the mostrecent calibration. In some embodiments in which memory devices 104 inrespective ranks in the same slice 110 are situated in physicalproximity such that they experience substantially the same voltage andtemperature drift, the same update 408 is applied to the calibrationdata for each of the registers 220: because the update 408 accounts forvoltage and temperature drift, and the memory devices 104 thatcorrespond to the registers 220 experience substantially the samevoltage and temperature drift, the same update 408 may be used for allof the registers 220. As shown in FIG. 4A, the same update 408 isprovided to each of the adders 404, each of which adds the update 408 tothe calibration data 216 in the corresponding register 220. In someembodiments, the update 408 is generated by accumulating timingadjustment data based on feedback from one or more memory devices 104and is provided to the plurality of storage elements 400 by thecalibration circuitry 230 (FIGS. 2A-2C), or alternatively by the edgetracking circuitry 226 (FIGS. 2A-2B).

FIG. 4B is a block diagram illustrating a plurality of storage elements420 for storing calibration data in accordance with other embodiments.The plurality of storage elements 400 is another example of theplurality of storage elements 218 (FIGS. 2A-2C). In the example of FIG.4B, the registers 220 a through 220 n are replaced with counters 422 athrough 422 n. An update 428 (e.g., based on timing adjustment data fromthe edge tracking circuitry 226 or calibration circuitry 230) isprovided to the storage elements 400 in the form of up/down data thatspecifies how much to increment or decrement each of the counters 422.Alternatively, update 428 specifies only whether to increment by one, ordecrement by one, the value in each of the counters 422. In yet anotheralternative, update 428 has a value selected from the set consisting of−1, 0 and +1, and thus specifies whether to decrement by one, leaveunchanged, or increment by one, the values in each of the counters 422.Up/down circuitry 424 receives the update 428 and increments ordecrements the counters 422 accordingly. As in FIG. 4A, the calibrationdata 216 for each of the memory devices 104 in a slice 110 are adjustedby the same amount based on the same update 428, which is determinedbased on feedback from one or more of the memory devices 104.

FIG. 5 is a circuit diagram of edge tracking circuitry 500 asimplemented using bang-bang CDR logic including an Alexander phasedetector, in accordance with some embodiments. FIG. 5 is merely oneexample of an implementation of the edge tracking circuitry 226 (FIGS.2A-2B); other implementations (e.g., using different CDR logic) arepossible. The EDC signal 228 is supplied to a first flip-flop 508 and asecond flip-flip 510. The first flip-flop is clocked by a data clock 504(e.g., the second clock signal 214, FIGS. 2A-2B) and the secondflip-flop is clocked by an edge clock 506 that is offset with respect tothe data clock 504 by an amount corresponding to a difference betweenthe center and edge of the EDC signal 228. For example, in a single datarate (SDR) system, the data clock 504 and edge clock 506 are out ofphase by 180°, while in a double data rate (DDR) system, the data clock504 and edge clock 506 are out of phase by 90°. The first and secondflip-flops 508 and 510 are coupled to third and fourth flip-flops 512and 514 and XOR gates 516 and 518 as shown, in an arrangement known asan Alexander phase detector. The outputs 520 and 522 of the XOR gates516 and 518 provide an indication of whether the EDC signal is early orlate and direct a counter 524 to either increment or decrement its valueaccordingly. The value of the counter 524, as filtered by a low-passfilter (LPF) 526, is the timing adjustment data 224 (FIGS. 2A-2B). Insome embodiments, the LPF 526 filters the counter 524 by filtering outone or more least-significant bits of the value in the counter 524. Theflip-flops 508 and 510 are examples of the flip-flops 652 and 646 (FIG.6A). The flip-flops 512 and 514, XOR gates 516 and 518, counter 524, andLPF 526 constitute CDR logic 502, which is an example of the CDR logic644 (FIG. 6A)

FIG. 6A illustrates data communication circuitry 600 in a memorycontroller 102 in accordance with some embodiments. A PLL 602, which isan example of a clock generator 202 (FIGS. 2A-2C, 3A-3B) generates afirst clock signal (“Data ck”) 612 that is transmitted to a memorydevice 104 (e.g., as illustrated in FIG. 6B) in a selected rank 106 of aslice 110. The first clock signal as generated by the PLL 602 also isprovided to a clock divider 604, which divides the first clock signal(e.g., by a factor of four). The divided clock signal 608 is transmittedto the memory device 104 and is provided to a flip-flop 606 to timetransmission of C/A signals 610 to the memory device 104.

The circuitry 600 includes one instance of timing circuitry 614 for eachdata line in a lane 108, and thus for each bit. The data lines in thelane 108 constitute a DQ bus 634. In the timing circuitry 614, a datatransmission phase mixer 616 receives the first clock signal from thePLL 602 and provides an intermediate clock signal to a DCDL 618, whichprovides a second clock signal to an output multiplexer (“OMux”) 622.The OMux 622 serializes write data 620 and drives the serialized writedata onto a data line of the DQ bus 634, via an amplifier 624, inaccordance with the second clock signal from the DCDL 618. The OMux 622thus is synchronized to the second clock signal from the DCDL 618, whichtimes transmission of the write data 620 to the memory device 104. Thedata transmission phase mixer 616 is an example of the phase mixer 208(FIG. 2A) and the DCDL 618 is an example of the DCDL 212 (FIG. 2A). Thedata transmission phase mixer 616 and DCDL 618 together are an exampleof the timing circuit 302 in FIG. 3A, and the OMux 622, together withthe amplifier 624, is an example of the transmitter 306 in FIG. 3A.

Also in the timing circuitry 614, a data receiver phase mixer 626receives the first clock signal from the PLL 602 and provides anintermediate clock signal to a DCDL 628, which provides a second clocksignal to a flip-flop 630. The flip-flop 630 samples data received froma data line of the DQ bus 634 as amplified by an amplifier 632. Thissampling is performed in accordance with the second clock signal fromthe DCDL 628. The flip-flop 630 thus is synchronized to the second clocksignal from the DCDL 628, which times receipt of read data from thememory device 104. The data receiver phase mixer 626 is an example ofthe phase mixer 208 (FIG. 2A) and the DCDL 628 is an example of the DCDL212 (FIG. 2A). The data receiver phase mixer 626 and DCDL 628 togetherare an example of the timing circuit 302 in FIG. 3B, and the flip-flop630 is an example of the sampling circuit 312 in FIG. 3B.

The circuitry 600 connects to an EDC signal line 654 that receives anEDC signal 228 (FIGS. 2A-2C) from the memory device 104 in the selectedrank 106 and the slice 110. The received EDC signal 228 is provided to afirst flip-flop 652 and a second flip-flop 646, which correspond to thefirst and second flip-flops 508 and 510 (FIG. 5). The first flip-flop652 is clocked by a clock signal generated by a data receiver phasemixer 648 together with a DCDL 650, while the second flip-flop 646 isclocked by a clock signal generated by an edge receiver phase mixer 640together with a DCDL 642. These clock signals are offset as describedfor the data and edge clocks 504 and 506 (FIG. 5). The phase mixers 640and 648 both receive as input the first clock signal generated by thePLL 602. The first and second flip-flops 652 and 646 provide theiroutput to CDR logic circuitry 644 (e.g., CDR logic 502, FIG. 5), whichgenerates timing adjustment data and provides the timing adjustment datato the DCDLs 618, 628, 642, and 650. In some embodiments, the CDR logiccircuitry 644 includes a low-pass filter (e.g., LPF 526, FIG. 5) tofilter the timing adjustment data. The phase mixers 640 and 648, DCDLs642 and 650, CDR logic 644, and flip-flops 646 and 652 togetherconstitute an example of the edge tracking circuitry 226 (FIGS. 2A-2B).

The circuitry 600 includes per-rank phase storage elements 656, whichare examples of the storage elements 218 (FIGS. 2A-2C). In someembodiments, the storage elements 656 are implemented as shown for thestorage elements 400 (FIG. 4A) or 420 (FIG. 4B). The storage elementsprovide rank-specific calibration data 658 to each of the phase mixers616, 626, 640, and 648. The phase mixers 616, 626, 640, and 648 thusadjust the phase of the first clock signal from the PLL 602 based on thecalibration data 658, and the DCDLs 618, 628, 642, and 650 adjust thephase of the clock signal output by respective phase mixers 616, 626,640, and 648 based on timing adjustment data from the CDR logic 644. Insome embodiments, the storage elements 656 include separate storageelements for read and write calibration data, such that the datatransmission phase mixer 616 receives calibration data 658 that isseparate from the calibration data received by the data receiver phasemixers 626 and 648 and edge phase mixer 640.

FIG. 6B illustrates data communication circuitry 660 in a memory device104 (FIG. 1) in accordance with some embodiments. The clock signal 608from the memory controller 102 is divided by a divider 662 and providedto the memory core to time operations within the memory device 104. TheC/A signal 610 from the memory controller 102 is sampled by a samplingcircuit 664, as clocked by the clock signal 608, and provided to thecore for processing. The first clock signal 612 from the memorycontroller 102 is distributed, via clock distribution circuitry 666, tocircuitry including a sampling circuit 668, an output multiplexer 670, acycle redundancy check (CRC) generator 676, and a multiplexer 680. Thesampling circuit 668, as clocked by the clock signal 612, samples writedata received from a respective data line of the DQ bus 634 and forwardsthe sampled write data 672 to the memory core. The output multiplexer670, as clocked by the clock signal 612, serializes read data 674received from the memory core and transmits the serialized read dataonto the respective data line of the DQ bus 634. The multiplexer 678provides either the read data 674 or the sampled write data 672 to theCRC generator 676, which generates a CRC signal 679 from the provideddata. The CRC generator 676 is clocked by the clock signal 612. Themultiplexer 680 transmits either the CRC signal 679 or the clock signal612 (e.g., when the memory device 104 is selected but idle) as the EDCsignal 654. FIG. 6B thus illustrates that the memory device 104 uses aclock signal 612 received from the memory controller 102 to timecommunications with the memory controller 102, and does not include aPLL or DLL.

Attention is now directed to methods of controlling memory devices. FIG.7A is a flow diagram illustrating a method 700 of controlling memorydevices in accordance with some embodiments. The method 700 is performed(702) at (and by) a memory controller 102 (FIG. 1) coupled to memorydevices 104 in a plurality of ranks 106, including a first memory devicein a first rank (e.g., rank 106 a, FIG. 1) and a second memory device ina second rank (e.g., rank 106 b, FIG. 1). The memory devices 104,including first and second memory devices 104, are situated in the sameslice 110.

In the method 700, a first clock signal is generated (704). In someembodiments, the first clock signal includes (706) a first plurality ofphase vectors. For example, the clock generator 202 (FIGS. 2A-2C, 3A-3B)generates a clock signal 204 that includes a plurality of phase vectors.

A phase of the first clock signal is adjusted (708) to generate a secondclock signal (e.g., Ck 214, FIGS. 2A-2B; clock signal 210, FIG. 2C),based on calibration data associated with the second memory device 104and timing adjustment data associated with feedback from at least thefirst memory device. For example, the phase of the first clock signal isadjusted based on calibration data stored in a respective storageelement of the plurality of storage elements 218 (FIGS. 2A-2C) thatcorresponds to the second memory device 104, and also based on timingadjustment data 224 (FIGS. 2A-2B) calculated based on the EDC signal 228received from the first memory device 104, with which the memorycontroller was communicating prior to communicating with the secondmemory device 104. The timing adjustment data 224 based on feedback fromthe first memory device 104 thus is applied to communication with thesecond memory device 104 after the controller 102 switches fromcommunicating with the first memory device 104 to communicating with thesecond memory device 104. In some embodiments, the feedback includes(710) feedback from the first and second memory devices 104. Forexample, after the controller 102 switches from communicating with thefirst memory device 104 to communicating with the second memory device104, the timing adjustment data 224 is based both on the feedback fromthe first memory device 104 and newly received feedback (e.g., in theform of the EDC signal 228) from the second memory device 104. Inanother example, the phase of the first clock signal is adjusted basedon calibration data stored in a respective storage element of theplurality of storage elements 218 (FIG. 2C), as updated based on thetiming adjustment data.

Communications with the second memory device 104 are timed (712) inaccordance with the second clock signal (e.g., Ck 214, FIGS. 2A-2B;clock signal 210, FIG. 2C). In some embodiments, a transmitter (e.g.,transmitter 306, FIG. 3A; OMux 622, FIG. 6A) transmitting data (e.g.,data 308/310, FIG. 3A) to the second memory device is clocked (714) withthe second clock signal (e.g., clock signal 304, FIG. 3A). In someembodiments, a sampling circuit (e.g., sampling circuit 312, FIG. 3B;flip-flop 630, FIG. 6A) receiving data (e.g., data 314/316, FIG. 3B)from the second memory device is clocked with the second clock signal(e.g., clock signal 304, FIG. 3B).

FIG. 7B is a flow diagram illustrating a method 720 of adjusting thephase of the first clock signal in the operation 708 of the method 700(FIG. 7A), in accordance with some embodiments. In the method 720, afirst phase adjustment of the first clock signal is performed (722)based on the calibration data (e.g., calibration data 216, FIGS. 2A-2B)associated with the second memory device 104, to generate anintermediate clock signal (e.g., intermediate clock signal 210, FIG. 2A,or 244, FIG. 2B). In some embodiments in which the first clock signalincludes (706, FIG. 7A) the first plurality of phase vectors, performingthe first phase adjustment includes mixing (724) phase vectors of thefirst plurality of phase vectors based on the calibration dataassociated with the second memory device. For example, the phase mixer208 (FIG. 2A) or 242 (FIG. 2B) mixes phase vectors of the firstplurality of phase vectors. In some embodiments, in which theintermediate clock signal includes a second plurality of phase vectors(e.g., the phase vectors of the intermediate clock signal 244, FIG. 2B),generating the second clock signal includes generating the secondplurality of phase vectors (726).

A second phase adjustment of the intermediate clock signal is performed(728) based on the feedback from at least the first memory device 104(e.g., from both the first and second memory devices 104) to generatethe second clock signal. In some embodiments, performing the secondphase adjustment includes delaying (730) the intermediate clock signalby an amount based on the timing adjustment data associated with thefeedback. For example, the DCDL 212 (FIG. 2A) delays the intermediateclock signal 210 based on the timing adjustment data 224. Alternatively,in some embodiments in which the intermediate clock signal includes(726) a second plurality of phase vectors, phase vectors of the secondplurality of phase vectors are mixed (732) based on the timingadjustment data associated with the feedback. For example, the phasemixer 246 (FIG. 2B) mixes phase vectors of the plurality 244 based onthe timing adjustment data 224 to generate the second clock signal.

FIG. 7C is a flow diagram illustrating a method 740 to be performedalong with the method 700 (FIG. 7A), in accordance with someembodiments. In the method 740, an amount to adjust calibration dataassociated with the first memory device 104 is determined (742), basedon feedback from the first memory device 104. The calibration dataassociated with the first memory device is adjusted (744) by thedetermined amount. The calibration data associated with the secondmemory device is also adjusted (746) by the determined amount. Forexample, as illustrated in FIGS. 4A and 4B, the calibration data storedin each of the registers 220 a through 220 n (FIG. 4A), or in each ofthe counters 422 a through 422 n (FIG. 4B), are updated by the sameamount, which may be determined based on feedback from a single memorydevice 104. After the method 740 has been performed, the adjustedcalibration data associated with the second memory device 104 is used inthe operation 708 of the method 700 (FIG. 7A).

In some embodiments, when the memory controller subsequentlycommunicates with the second memory device 104, additional timingadjustments are determined based on newly received feedback from thesecond memory device 104 and the second timing is modified based on theadditional timing adjustments. For example, the DCDL 212 (FIG. 2A) orphase mixer 246 (FIG. 2B) makes the additional timing adjustments.Alternatively, the calibration data 216 for the second memory device 104is further updated based on the additional timing adjustments.

While the methods 700, 720, and/or 740 include a number of operationsthat appear to occur in a specific order, it should be apparent that themethods 700, 720, and/or 740 can include more or fewer operations, someof which can be executed serially or in parallel. An order of two ormore operations may be changed and two or more operations may becombined into a single operation.

FIG. 7D is a flow diagram illustrating a method 760 of controllingmemory devices 104 in accordance with some embodiments. The method 760is performed (762) at and by a memory controller 102 (FIG. 1) coupled tomemory devices 104 in a plurality of ranks 106, including a first memorydevice 104 in a first rank (e.g., rank 106 a, FIG. 1) and a secondmemory device 104 in a second rank (e.g., rank 106 b, FIG. 1). Thememory devices 104, including first and second memory devices 104, aresituated in the same slice 110.

In the method 760, a calibration of first timing (e.g., transmit timingor receive timing, or write or read timing) is performed (764) forcommunication between the memory controller 102 and the first memorydevice 104. A calibration of second timing (e.g., transmit timing orreceive timing) is performed (766) for communication between the memorycontroller 102 and the second memory device 104. These calibrations areperformed, for example, using the calibration circuitry 230 (FIGS.2A-2C), and the resulting calibration data is stored in the plurality ofstorage elements 218 (FIGS. 2A-2C). In some embodiments, thesecalibrations are performed upon powering on the memory controller 102,after a reset operation, and/or after no communication has occurredbetween the memory controller 102 and the first and second memorydevices 104 for a specified period of time.

Communication with the first memory device 104 is performed (768) inaccordance with the first timing (e.g., in accordance with thecalibration data 216 for the first memory device 104 as stored in arespective storage element of the plurality 218 and provided to thephase mixer 208 or 242, FIGS. 2A-2C). While communicating with the firstmemory device 104, timing adjustments are determined based on feedbackfrom the first memory device 104 and the first timing is modified basedon the timing adjustments. For example, the edge tracking circuitry 226(FIGS. 2A-2B) determines timing adjustment data 224, and the DCDL 212(FIG. 2A) or phase mixer 246 (FIG. 2B) makes timing adjustments usingthe timing adjustment data 224. The timing adjustments are thus based onfeedback from the first memory device 104. Alternatively, updates to thecalibration data 216 for the first memory device 104 are made (e.g., bythe calibration circuitry 230 or edge tracking circuitry 226) based onfeedback from the first memory device 104.

In some embodiments, communicating with the first memory device 104includes writing data (770) to the first memory device 104 (e.g., usingthe transmitter 306, FIG. 3A). In some embodiments, communicating withthe first memory device 104 includes reading data (772) from the firstmemory device 104 (e.g., using the sampling circuit 312, FIG. 3B).

Communication with the second memory device 104 is performed (774) inaccordance with the second timing as modified by the timing adjustments.For example, the communication is performed in accordance with thecalibration data 216 for the second memory device 104 as stored in arespective storage element of the plurality 218 and provided to thephase mixer 208 or 242 (FIGS. 2A-2C). In some embodiments, the DCDL 212(FIG. 2A) or phase mixer 246 (FIG. 2B) makes the timing adjustmentsbased on the timing adjustment data 224. Alternatively, the calibrationdata 216 for the second memory device 104 has been updated based ontiming adjustment data for the first memory device 104. Whilecommunicating with the second memory device 104, additional timingadjustments are determined based on newly received feedback from thesecond memory device 104 and the second timing is modified based on theadditional timing adjustments. For example, the DCDL 212 (FIG. 2A) orphase mixer 246 (FIG. 2B) makes the additional timing adjustments.Alternatively, the calibration data 216 for the second memory device 104is further updated based on the additional timing adjustments.

In some embodiments, communicating with the second memory device 104includes writing data (776) to the second memory device 104 (e.g., usingthe transmitter 306, FIG. 3A). In some embodiments, communicating withthe second memory device 104 includes reading data (778) from the secondmemory device 104 (e.g., using the sampling circuit 312, FIG. 3B).

In some embodiments, the feedback from the first and second memorydevices 104 includes respective EDC signals 228 (FIGS. 2A-2C) from thefirst and second memory devices 104. In some embodiments, the feedbackfrom the first and second memory devices 104 includes respective readstrobes from the first and second memory devices 104. The feedback fromthe first and second memory devices 104 varies based on voltage andtemperature changes for the first and second memory devices 104, andthus allows the controller 102 to update its timing based on voltage andtemperature variations.

While the method 760 includes a number of operations that appear tooccur in a specific order, it should be apparent that the method 760 caninclude more or fewer operations, which can be executed serially or inparallel. An order of two or more operations (e.g., operations 764 and766) may be changed and two or more operations may be combined into asingle operation.

The foregoing description, for purpose of explanation, has beendescribed with reference to specific embodiments. However, theillustrative discussions above are not intended to be exhaustive or tolimit the inventions to the precise forms disclosed. Many modificationsand variations are possible in view of the above teachings. Theembodiments were chosen and described in order to best explain theprinciples of the inventions and their practical applications, tothereby enable others skilled in the art to best utilize the inventionsand various embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A memory system, comprising: a plurality ofmemory devices in respective ranks, including a first memory device in afirst rank and a second memory device in a second rank; and a memorycontroller comprising: a clock generator to generate a first signal; anda timing circuit to generate a second signal from the first signal totime data transfer communications with the first and second memorydevices, the timing circuit to adjust a phase of the first signal togenerate the second signal when the memory controller transitions from adata transfer communication with the first memory device to a datatransfer communication with the second memory device, the second signalhaving a phase that is based on calibration data associated with thesecond memory device, wherein the calibration data is to be establishedbased on a strobe signal received from the second memory device.
 2. Thememory system of claim 1, comprising a module with first and secondopposing sides, wherein the first memory device is situated on the firstside of the module and the second memory device is situated opposite tothe first memory device on the second side of the module.
 3. The memorysystem of claim 1, wherein the first and second memory devices aresituated in a common package.
 4. The memory system of claim 1, whereinthe timing circuit is configured to adjust the phase of the firstsignal, when the memory controller transitions from a data transfercommunication with the first memory device to a data transfercommunication with the second memory device, based on calibration dataassociated with the second memory device and timing adjustment dataassociated with feedback from at least the first memory device.
 5. Thememory system of claim 4, wherein the feedback includes strobe signalsreceived from both the first memory device and the second memory device.6. The memory system of claim 1, wherein the memory controller includesedge tracking circuitry to generate timing adjustment data associatedwith respective memory devices based on feedback from the respectivememory devices, the timing adjustment data associated with the secondmemory device to be used to update the calibration data associated withthe second memory device.
 7. The memory system of claim 1, wherein thetiming circuit comprises: a first phase adjuster to adjust the phase ofthe first signal; and a plurality of respective storage elements, eachto store calibration data associated with a respective memory device ina respective rank, wherein one of the respective storage elements is tostore the calibration data associated with the second memory device;wherein the first phase adjuster is to be selectively coupled to thestorage element storing the calibration data associated with the secondmemory device when the second signal is being generated to time datatransfer communications with the second memory device, the first phaseadjuster to adjust the phase of the first signal based on thecalibration data stored in the selectively coupled storage element. 8.The memory system of claim 7, further comprising: calibration circuitryto perform calibration for the first memory device and, in response, todetermine an amount by which to adjust calibration data associated withthe first memory device in a first storage element of the plurality ofstorage elements; and adjustment circuitry to adjust calibration dataassociated with the second memory device in a second storage element bythe determined amount.
 9. The memory system of claim 8, wherein theadjustment circuitry is to adjust calibration data in each respectivestorage element by the determined amount.
 10. The memory system of claim7, wherein: the first phase adjuster is to generate an intermediatesignal by adjusting the phase of the first signal; and the timingcircuit further comprises a second phase adjuster to generate the secondsignal by adjusting a phase of the intermediate signal based on thetiming adjustment data derived from the feedback from at least the firstmemory device.
 11. The memory system of claim 10, wherein the secondphase adjuster is to operate using values of the timing adjustment dataassociated with communicating with the first memory device duringcommunication with the first memory device, and to continue to operateusing a value of the timing adjustment data associated withcommunicating with the first memory device initially after the memorycontroller transitions from communicating with the first memory deviceto the second memory device.
 12. The memory system of claim 10, whereinthe second phase adjuster comprises a digitally controlled delay line.13. The memory system of claim 1, further comprising a transmitter,coupled to the timing circuit, to transmit data to the second memorydevice in accordance with the second signal.
 14. The memory system ofclaim 1, further comprising a sampling circuit, coupled to the timingcircuit, to sample data from the second memory device in accordance withthe second signal.
 15. A method of controlling memory devices,comprising: at a memory controller coupled to memory devices in aplurality of ranks, including a first memory device in a first rank anda second memory device in a second rank: generating a first signal;generating timing adjustment data associated with respective memorydevices in the first rank based on feedback from the respective memorydevices in the first rank, wherein the feedback includes a first strobereceived from at least the first memory device; adjusting a phase of thefirst signal to generate a second signal, the adjusting based oncalibration data associated with the second memory device; and timingdata transfer communications with the second memory device, inaccordance with the second signal.
 16. The method of claim 15, furthercomprising: storing, in a plurality of respective storage elements,calibration data associated with respective memory devices in respectiveranks, including storing in one of the respective storage elements thecalibration data associated with the second memory device; selectivelycoupling a first phase adjuster to the storage element storing thecalibration data associated with the second memory device when thesecond signal is being generated to time data transfer communicationswith the second memory device, to adjust the phase of the first signalbased on the calibration data stored in the selectively coupled storageelement.
 17. The method of claim 16, further comprising: performingcalibration for the first memory device to determine an amount by whichto adjust calibration data associated with the first memory device in afirst storage element of the plurality of storage elements; andadjusting calibration data associated with the second memory device in asecond storage element by the determined amount.
 18. The method of claim16, including generating an intermediate signal by adjusting the phaseof the first signal, and generating the second signal by adjusting aphase of the intermediate signal based on the timing adjustment dataderived from the feedback from at least the first memory device.
 19. Themethod of claim 18, wherein adjusting the phase of the intermediateincludes adjusting the phase of the intermediate using values of thetiming adjustment data associated with communicating with the firstmemory device during communication with the first memory device, andcontinuing to adjust the phase of the intermediate using values of thetiming adjustment data associated with communicating with the firstmemory device during communication with the first memory deviceinitially after transitioning from communicating with the first memorydevice to the second memory device.
 20. The method of claim 15, furthercomprising transmitting data to any of the plurality of memory devicesin the respective ranks in accordance with the second signal.
 21. Themethod of claim 15, wherein adjusting the phase of the first signal togenerate the second signal includes adjusting the phase of the firstbased on calibration data associated with the second memory device andtiming adjustment data associated with feedback from at least the firstmemory device.
 22. A memory controller to control the operation of aplurality of memory devices in respective ranks, including a firstmemory device in a first rank and a second memory device in a secondrank, the memory controller comprising: a clock generator to generate afirst signal; and a timing circuit to generate a second signal from thefirst signal to time data transfer communications with the first andsecond memory devices, the timing circuit to adjust a phase of the firstsignal to generate the second signal when the memory controllertransitions from a data transfer communication with the first memorydevice to a data transfer communication with the second memory device,the second signal having a phase that is based on calibration dataassociated with the second memory device, wherein the calibration datais to be established based on a strobe signal received from the secondmemory device.